The problem of formation damage under the impact of a drilling mud (or a flush liquid) is highly important, particularly in the case of long horizontal wells because most horizontal wells are completed without casing, i.e., without production string cementing and perforating.
Drilling mud is a complex mix of clay, fine particles (from several millimeters to less than one micron), and organic additives (polymers, surfactants, etc.) contained in a “carrying” fluid (a mud base). Such a base fluid could be water, oil, or some synthetic fluid.
While drilling, mud filtrate together with fine particles and clay contained therein may penetrate the formation near a well bore under excess pressure caising a considerable permeability reduction in the affected area. This phenomenon is commonly referred to as “damage of the formation near-welbore area” or simply “formation damade.”
During a well clean-up technological procedure (gradual transfer to the production mode) these components are partially washed out of the mud damaged zone, which facilitates its permeability repairing. However, some of those components remain captured in the rock pore space (adsorption on the pore surface, capture in pore throats, etc.), which results in a significant difference between the initial permeability and the repaired permeability following the well clean-up procedure (the repaired permeability would normally represent about 50% to 70% of the initial permeability).
A laboratory method commonly used to control drilling mud quality is a filtration experiment in which the drilling mud is injected into a core sample with subsequent flowback (i.e., the displacement of the drilling mud in the sample by an original formation fluid), in which process measures are taken of permeability reduction/repairing as a function of the amount of fluid pore volumes (drill mud or formation fluid).
However, information on concentrations of clay and other mud components captured in the pore space after the mud has been displaced is important for understanding a mechanism behind formation damage and identifying an appropriate method for improving production index of well (minimizing the mud damaged zone). The conventional quality control method of the drilling mud referred to above envisions no such measurements.
Quantitative analysis of the formation damage mechanism associated with the penetration of clay materials while drilling is of the highest interest in view of the wide use of clay-based drilling muds. For instance, the Russian Federation National Standard ΓOCT 25795-83 recommends bentonite clays for drill mud preparation.
Weight concentration of drilling mud clay penetrating the pore space is typically small (no more than 1% or 1.5% by weight). Nonetheless, with a high swelling factor of clay and its porosity, even such a small weight concentration results in a significant reduction of rock permeability (5 to 20 times down).
The technical challenge is related to the fact that small weight concentrations of clay in a porous medium are hard to measure because X-ray diffraction and X-ray micro-computed tomography methods fail to provide sufficient resolution for weight concentrations of less than 1%.
U.S. Pat. Nos. 4,540,882 and 5,027,379 disclose methods for determining drill mud penetration depth using X-ray micro-computed tomography of core samples with a contrast agent added thereto. However, the use of a contrast agent dissolved in the “carrying” fluid does not provide determination of penetration and concentrations of clay and other weak-contrast admixtures contained in the drilling mud because the drill mud filtrate penetration depth generally differs from that of the said admixtures.
U.S. Pat. No. 5,253,719 offers a method for identification of formation damage mechanism through analysing radial-oriented core samples taken from the well. Core samples are analyzed with the help of various analytical methods identifying the type and extent of formation damage and depth of the mud-damaged zone. The analytical methods used include a X-Ray Diffraction (XRD) method, a Local X-Ray Spectral Analysis, Scanning Electron Microscopy (SEM), Back-Scattering Electron Microscopy, Petrography Analysis, and Optical Microscopy.